One example of the related prior art is found in the pulsed arc welding system disclosed in Japanese Patent Application Laid Open No. 19177-1982 and another such example is found in the short circuiting transfer arc welding system disclosed in Japanese Patent No. 54585-1987.
The pulsed arc welding system in Japanese Patent Application Laid Open No. 19177-1982 performs a welding process by executing the following procedures. First, the system generates a pulsed arc electric current between a consumable welding wire electrode (hereinafter simply referred to as the "wire electrode") and the base metal. Then the system fuses the base metal and the wire electrode by applying the heat generated by a pulsed arc of electric current. Next, the system cuts off the tip of the molten wire electrode by applying the electromagnetic pinching force of such a pulsed arc electric current to the tip of the wire electrode. Finally, the system intermittently transfers molten globules of the wire electrode to the base metal (this transfer is called "spraying transfer". The advantage of this system are as follows. First, the system is capable of performing a welding process with a pulsed electric current in an area where this welding has an average electric current lower than that of a direct current arc welding system. Second, the system can perform the welding process to a thinner base metal. Third, the system can attain the spraying transfer thereby eliminating the spatter which would otherwise occur in the course of welding.
Also, the short circuiting transfer arc welding system disclosed in Japanese Patent No. 54585 performs a similar welding process. The welding system periodically generates an arc of electric current between the wire electrode and the base metal. The heat generated by this are of electric current melts, the base metal and the wire electrode. Subsequently, the welding system transfers the molten globule formed on the tip of the wire electrode to the base metal by a short circuiting transfer. Consequently, this system is capable of securing a stable welding state by periodically generating an arc of electrical current, and then transferring the molten globule by short circuit transfer.
However, in order to achieve high-quality welding by the pulsed arc welding process, it is necessary to eliminate the spatter which tends to occur in the molten globule during the welding process. In addition, it is necessary to prevent the occurrence of undercuts (i.e., defects in the shape of the welding beads) and to form separated molten globules which are approximately identical size.
In order to eliminate the spatter, it is necessary to prevent the wire electrode and the base metal from touching each other (i.e., short circuiting). In order to prevent the occurrence of undercuts, it is necessary to shorten the arc length. Both of these requirements can be satisfied if the molten globules are formed into fine particles when they are separated from the wire electrode during the spraying transfer. Moreover, in order to create separated molten globules that are substantially the same size, the same pulse form should be repeated periodically in the waveform of the pulsed arc electric current.
The operation of conventional pulsed arc welding systems are shown in FIG. 54. In FIG. 54, .tau. expresses the pulse width, I.sub.B expresses the base electric current, and I.sub.f represents the pulsed peak electric current.
FIG. 54(a) illustrates the operation of a conventional welding system in an atmosphere comprising 100 percent CO.sub.2 gas. In this atmosphere, the width .tau. of the pulsed arc is narrow in relation to the molten globule on the wire electrode. FIG. 54(b) illustrates the operation of a conventional welding system in an atmosphere comprising argon gas and 20 percent CO.sub.2 gas. In this atmosphere, the width .tau. of the pulsed arc is relatively wide in relation to the molten globule on the wire electrode. The welding systems illustrated in FIG. 54(a) and FIG. 54(b) cannot perform high quality welding. In the welding system shown in FIG. 54(b), the base electric current I.sub.B is set at a high level and the pulse width .tau. is narrow. Consequently, the molten globule at the tip of the wire electrode cannot be separated until the molten globule changes from its shape in the state P.sub.0 into its shape in the state P.sub.a1 and further into a large-sized lump shown in the state P.sub.a2.
In the welding system shown in FIG. 54(b), the base electric current I.sub.B is set at a low level and the pulse width .tau. is wide. Consequently, the pulsed electric current exerts an electromagnetic force F in the upward direction, the molten globule on the tip of the wire electrode changes from the shape in state P.sub.0 into the shape in state P.sub.b in other work, the shape of the molten globule constricts and the molten globule is elevated and begins to rotate at a high velocity. As a result of this elevation, two consequences may occur. First, the rotating molten globule may separate from the wire electrode and scattered as spatter over areas other than the base metal as shown in state. Second, the molten globule may stick again to the wire electrode as in the state Pb.sub.2.
Other disadvantages of the conventional pulsed arc welding equipment occur depending on the value of the pulsed peak electric current I.sub.P. When the value of the pulsed peak electric current I.sub.P is set at a low level, the molten globule formed at the tip of the wire electrode is only slightly lifted up by the pulses, so that the molten globule cannot be separated until it grows into a large-sized drop. Consequently, the growth of the molten globule formed on the tip of the wire electrode into a large-sized drop results in the formation of a short circuit between the molten globule and the base metal. As a result, a lot of spatter is scattered in the area around the weldment during the welding process or (i.e. is a defect in the welding beads) is formed. On the other hand, setting the value of the pulsed peak electric current I.sub.P at a high level will require an increased capacity of the power source unit for the welding equipment. This requirement increases the weight and cost of the welding equipment.
In an attempt to overcome these disadvantages, the present inventors filed, prior to the present invention, applications for patents, No. 309388-1987 and No. 265083-1988, as published in Japanese Patent Laid Open No. 254385-1989. These applications taught a pulsed arc welding system which produces fine particles of molten globules during the process of transferring them to the base metal. Also, the welding system performs the transfer of the molten globules in an orderly manner by moderating the force that lifts up the molten globule formed on the tip of a wire electrode. The welding system moderates the force by dividing one pulsed electric waveform into a plurality of groups formed of pulsed electric currents (i.e. pulse groups). These pulsed electric currents as wide as one or more pulse widths are arranged in one or more types of pulse intervals. These pulse groups are repeated in every period, and a discharge electric current waveform is obtained by duplicating a continuous base electric current over such pulse groups.
However, in the event the arc welding is performed as the wire electrode is being moved in a constant direction over the base metal, the magnetic field which is formed in the welding space of this pulsed arc welding system has a different distribution which depends on the paths of the electric current flowing from the welding torch to the arc and from the arc to the base metal. In other words, the distribution of the magnetic field in the welding space will be different from one case to another, depending on the differences in the shape of welded joints and the differences in the ground points. The electromagnetic force is exerted on the arc, depending on the distribution of the magnetic field and the direction of the arc current, and causes a magnetic arc blow. A magnetic arc blow is a phenomenon in which the magnetic force makes the arc lean in relation to the base metal.
A magnetic arc blow makes it utterly impossible to perform any welding work in a favorable condition. One reason is that it becomes difficult to perform any regular separation of molten globules due to an extension which takes place in the length of the arc as the molten globule is lifted up by the deflecting arc. This problem is shown in (A-1) through (C-1) and (A-3) through (C-3) in the illustrations of the individual processes for the separation of the molten globule presented in FIG. 55. As a result of the extension of the arc the separated molten globule is flung into the area outside the welding beads.
Also, the short circuiting transfer arc welding system will similarly have a disturbance in the period for the repetition of the short circuiting and the arcing. This is due to a change in the timing of the short circuiting of the molten globule when the molten globule is formed and growing on the tip of the wire electrode. As shown in S.sub.1a through S.sub.3a in FIG. 56, the growing molten globule is pushed upward by a deflection of the arc by a magnetic arc blow. As the result of such a disturbance, there are problems such as the formation of irregularities on the welding beads, fluctuations in the depth of weld penetration, and a resultant failure in securing sufficient strength in the weld.
The present invention has been designed to overcome the various problems described above, and it is an object of the present invention to offer a pulsed arc welding system which is capable of performing arc welding in a reliable manner by preventing irregular growth in the molten globule due to factors such as the phenomenon known as magnetic arc blow and external disturbances and also by securing the regular transfer of the molten globule in the direction of the welding beads.